Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers.
A personal audio device may include a polymorphic playback system. A polymorphic playback system may include a playback system in which one or more parameters of a signal path of the playback system are varied based on one or more characteristics of a playback signal processed by the signal path. Examples of a polymorphic playback system include a dynamic range enhancement playback system in which signal gains of a playback path are varied based on one or more characteristics (e.g., signal magnitude) of the playback signal, a multiple processing path playback system in which a processing path for processing the playback signal is selected based on one or more characteristics (e.g., signal magnitude) of the playback signal, an amplifier with a configurable output stage (e.g., configurable as a Class AB output stage or Class D output stage) in which the configuration of the output stage is based on one or more characteristics (e.g., signal magnitude) of the playback signal, or any other suitable polymorphic playback system.
Many playback systems, such as those that perform adaptive noise cancellation, are required to have low latency in their playback paths. In addition, many playback systems include playback signals with a significant amount of out-of-band noise. In some instances, the presence of out-of-band noise may cause switching of a polymorphic playback system from a first polymorphic mode in which a first set of one or more parameters are applied to the signal path based on one or more characteristics of a playback signal to a second polymorphic mode in which a second set of one or more parameters are applied to the signal path based on the one more characteristics of a playback signal, and vice versa. In some instances, such out-of-band noise may lead to frequent switching or oscillation between the polymorphic modes. Such frequent switching may occur because thresholds in a control subsystem for switching between modes may be optimized for a particular data source. However, if the data source is changed, or its noise performance drifts over time, noise fluctuation may cause random crossing of the switching threshold and thus, oscillation between polymorphic modes.
Such out-of-band noise may influence an in-band signal received by a level detector for detecting a playback signal magnitude in a polymorphic playback system that utilizes the playback signal magnitude as a characteristic for setting one or more parameters of the signal path of the playback system. While a strong low-pass filter could be used to filter out the out-of-band noise, use of a strong low-pass filter may add too much undesirable latency to signal detection.